The present invention is directed to the field of wireless telecommunications, with particular applicability to the detection and avoidance of sources of interference that can disrupt service over a wireless network. Many types of interference can be encountered in the operation of a Wireless Local Area Network (WLAN), particularly WLANs that operate in the 2.4 GHz and 5 Ghz bands. This interference can be produced by other devices licensed by the FCC to operate in the 2.4 or 5 GHz band, such as Bluetooth components, military and aircraft radar systems, certain types of cordless telephones, home RF systems, and various old style radios, including those that “frequency hop” within the unlicensed bands. Such interference can result in packet drops between clients and access points and can thereby disrupt service in the WLAN.
The IEEE 802.11(h) standard requires network management for WLANs that compete over the 5 GHz band with interfering sources. The standard requires the network to detect licensed users of other frequencies. It is necessary that a WLAN take measures to insure that it doesn't interfere with other licensed sources operating on the band, i.e., aircraft radar, etc. For example, the APs can instruct their clients to change to a non-interfering channel, or steer away from interferers for clients equipped with directional antennas.
Various approaches have been employed in previous systems for detecting sources of interference. Within an AP's cell, one or more clients (or stations, STA) may be geographically distributed so as to have a reception range that extends beyond the cell, outside the reception range of the AP. The clients can thereby be used to detect and report to the AP sources of interference from outside the cell. The AP would then manage client connectivity so as to avoid the interfering sources. FIG. 4 shows a typical WLAN network with two APs each having four associated STAs. Real-time network management could be effected by keeping the clients continuously on-line, listening for interferers. However, this solution is not practical since most clients in a WLAN are mobile (e.g. notebook computers or personal digital assistants), and the client battery would be quickly drained since the high power energy detect circuits are left on all the time.
Another previous-type solution entails “active polling” of clients by the AP to locate sources of interference. Clients are permitted to shut down during periods of inactivity, when they are not communicating with the AP. This happens normally in WLAN where a STA with no packet activity will listen for and respond to beacon signals from the AP so they can remain associated to the network. In this regard, the client is periodically activated by an internal timer, in synch with the period of the beacon signal. The clients are each turned on to listen to beacon. Each client sends a response signal to the AP in reply to the beacon, after which they “go back to sleep” in order to conserve power. By such active polling, the clients maintain association with the network, since they are disassociated if they do not respond to the beacon within a specific interval. In this way, power is conserved since the clients are in a low-power “sleep” mode for most of the duty cycle which is beneficial especially since network traffic is quite bursty and most of the time STAs are not doing any packet activity.
In order to detect interferers, the beacon periodically includes a “power monitor” command which instructs the clients to monitor the band and report if any interfering energy is detected. The AP collects the data from the client stations and then determines if the energy is interference, meaning that it cannot be read as a packet encrypted in accordance with the 802.11 network. Such “active polling” schemes suffer from certain deficiencies. Though power is conserved as compared to continuous sampling, a large amount of power is still consumed. The beacon period is several milliseconds and so the clients must turn on and off several times per second. Also, power is consumed by sending a response signal with the beacon power monitor command. This also results in a lot of network traffic, placing additional service demands upon the entire network. Also the AP is heavily burdened with power calculations from all clients which may number into the 100s for a large network. This processing burden may result in significant overhead in the AP. Further, the AP power monitor commands are only issued on the order of once per second. It is possible to miss a lot of interfering energy between these power samples. Thus, energy and traffic demands remain high with active polling schemes and are not as effective with locating sources of interference, resulting in sub-optimal cost/benefit realization.